This invention relates to a process for improving the thickness uniformity of a thin oxide layer on a semiconductor wafer, and more particularly, to a process having two progressive thermal oxidation steps for forming a uniform thickness oxide layer on a semiconductor wafer, as well as to the semiconductor wafer thereby produced. As used herein, xe2x80x9csemiconductor waferxe2x80x9d means any microelectronic device, substrate, chip or the like, e.g., of silicon, used to provide an integrated circuit or other related circuitry structure, and in particular capable of forming an oxide layer in situ on a surface thereof.
In fabricating microelectronic semiconductor devices and the like on a semiconductor wafer (substrate or chip), e.g., of silicon, to form an integrated circuit (IC), etc., various metal layers and insulation layers are deposited in selective sequence, and in some cases oxide layers are grown in situ on the wafer. To maximize integration of device components in the available wafer area to fit more components in the same area, increased IC miniaturization is utilized. Reduced pitch dimensions are needed for denser packing of components per present day very large scale integration (VLSI), e.g., at sub-micron (below 1 micron, i.e., 1,000 nanometer or 10,000 angstrom) dimensions.
A typical conventional oxide layer formation process used in the IC fabrication of a semiconductor wafer involves a one-step thermal oxidation of a surface thereof in an oxidation furnace, e.g., of quartz. The wafer, e.g., at room temperature, i.e., about 20xc2x0 C., is loaded, e.g., progressively, via a boat containing a row of successive wafers, into the furnace which is maintained at a low loading temperature, e.g., of about 400-600xc2x0 C., and the temperature of the furnace is xe2x80x9cramped upxe2x80x9d, i.e., increased, along with the temperature of the wafer, to a high oxidizing temperature, e.g., of about 700-1200xc2x0 C. The wafer surface is then subjected to oxidation to grow in situ the oxide layer thereon. After the oxide layer is so grown, the furnace temperature is xe2x80x9cramped downxe2x80x9d, i.e., decreased, to a low unloading temperature, e.g., of about 400-600xc2x0 C., and the wafer is unloaded from the furnace for further processing.
In general, the oxidation furnace is operated 24 hours a day, and never cools down to room temperature. Instead, the furnace varies from a low loading or unloading temperature, e.g., of about 400-600xc2x0 C., to a high oxidizing temperature, e.g., of about 700-1200xc2x0 C. Hence, during the loading of the row of room temperature wafers into the furnace, the leading wafer is exposed to the heating conditions in the furnace significantly earlier than the trailing wafer, whereas the trailing wafer is correspondingly exposed to the exterior ambient air significantly longer than the leading wafer.
In particular, silicon dioxide (SiO2) is in most cases; grown in situ as a high temperature stable insulating oxide on silicon. The oxide thickness uniformity within a given wafer and from wafer to wafer, as well as the reproducibility, i.e., from run to run, of the desired target thickness is of great importance for modern semiconductor circuits with small design ground rules and small oxide thicknesses.
One major problem is to control significantly the interim time between a typically contemplated wafer precleaning step, e.g., chemical precleaning by etching with buffered HF solution, and the beginning of the in situ thermal oxidation. During this interim time, a native oxide grows in situ on the silicon wafer at room temperature followed by a low temperature oxide which grows during loading and during ramp up to the high oxidizing temperature.
The last processing step before the deposition of a thermal oxide, i.e., before the thermal oxidation for growing the oxide in situ, is usually a wafer precleaning step, as noted above. During this precleaning step, the previously grown native oxide, which forms on mere contact with air at room temperature, i.e., about 20xc2x0 C., is removed. Unfortunately, after the precleaning step, a new oxide layer starts growing on the silicon wafer as a native oxide at room temperature unless the precleaning step is performed in an inert atmosphere, i.e., under an inert ambient such as nitrogen, argon, or other inactive gas, and also as a low temperature oxide unless the wafer is loaded into the oxidation furnace without coming into contact with ambient air.
The thickness of the new oxide layer grown at room temperature depends mainly on the duration between the precleaning step and the incorporation, i.e., progressive loading, of the wafer into the oxidation furnace. In practice, this time is difficult to control in a fabrication environment. This is mainly because the leading wafer in the row of wafers being loaded into the furnace is exposed to the heating conditions in the furnace significantly earlier than the trailing wafer, whereas the trailing wafer is correspondingly exposed to the exterior ambient air significantly longer than the leading wafer. As a result, the oxide layer which is produced by the usual one-step thermal oxidation process is non-uniform in thickness and extent, and thus non-homogeneous in character.
It is desirable to have a process providing improved thickness uniformity of a thin oxide layer in semiconductor wafer fabrication, within the given wafer and from wafer to wafer, with reproducibility from run to run of the desired target thickness, which creates more ideal or uniform starting conditions for the thermal oxidation of the wafer surface so as to render less important or inconsequential any uncontrolled formation of an oxide layer during the interim time between a wafer precleaning step and the beginning of the thermal oxidation.
The foregoing drawbacks are obviated in accordance with the present invention by providing a two-step thermal oxidation process for improving the thickness uniformity of a thin oxide layer in semiconductor wafer fabrication. The improved thickness uniformity is attained within the given wafer and from wafer to wafer, with reproducibility from run to run of the desired target thickness, because the present invention creates more ideal or uniform starting conditions for the thermal oxidation of the wafer surface which render less important or inconsequential any uncontrolled formation of an oxide layer during the interim time between a wafer precleaning step and the beginning of the thermal oxidation.
The process comprises the main steps of:
loading a semiconductor wafer having a surface which is subject to formation of an oxide layer thereon but which is substantially oxide layer-free, into an oxidation furnace under an inert atmosphere while maintaining the furnace at a low loading temperature;
adjusting the temperature of the wafer in the furnace to a low oxidizing temperature while maintaining the wafer under an inert atmosphere;
subjecting the wafer to initial oxidation at the low oxidizing temperature sufficiently to form a substantially uniform initial thickness oxide layer on the wafer surface;
increasing the temperature of the furnace to a high oxidizing temperature while maintaining the wafer under an inert atmosphere;
subjecting the wafer to final oxidation at the high oxidizing temperature sufficiently to increase uniformly the thickness of the oxide layer to a selective final thickness; and
recovering the resultant substantially uniform final thickness oxide layer-containing wafer from the furnace.
More particularly, the recovering of the wafer includes the further steps of:
decreasing the temperature of the furnace to a low unloading temperature under an inert atmosphere sufficiently for unloading the wafer from the furnace without significant further oxidation of the wafer surface; and
unloading the resultant substantially uniform final thickness oxide layer-containing wafer from the furnace.
Desirably, the low loading temperature is about 400-600xc2x0 C., the low oxidizing temperature is about 400-600xc2x0 C., the high oxidizing temperature is about 700-1200xc2x0 C., and the low unloading temperature is about 400-600xc2x0 C. Typically, the wafer is subjected to oxidation at the low oxidizing temperature for about 15-30 minutes at about 1 atm pressure, then the temperature is increased from the low oxidizing temperature to the high oxidizing temperature over a period of about 15-30 minutes, and the wafer is subjected to oxidation at the high oxidizing temperature for about 5-15 minutes at about 1 atm pressure.
Advantageously, the oxidation at the low oxidizing temperature is carried out in the presence of dry oxygen, and the oxidation at the high oxidizing temperature is carried out in the presence of oxygen and/or water vapor.
In particular, the oxidation at the low oxidizing temperature is carried out sufficiently to form an oxide layer having an initial thickness of up to about 10 angstroms (i.e., 0.001 micron or 1 nm), and the oxidation at the high oxidizing temperature is carried out sufficiently to increase the oxide layer to a final thickness of about 20-100 angstroms (i.e., 0.002-0.01 micron or 2-10 nm). Desirably, the wafer comprises silicon and the oxide layer comprises silicon dioxide. Also, the surface of the wafer is conveniently precleaned, e.g., chemically precleaned by use of a buffered HF etching solution, to provide such surface in substantially oxide layer-free condition.
The present invention also contemplates the substantially uniform thickness oxide layer-containing semiconductor wafer which is produced by the above two-step progressive thermal oxidation process.
Advantageously, the two-step progressive thermal oxidation process of the invention creates a very uniform thin first oxide layer by a first low temperature oxidation process step followed by a second oxidation process step at a higher temperature to grow further the thin oxide layer to the desired final oxide uniform thickness.
The invention will be more readily understood from the following detailed description taken with the accompanying drawings and claims.